After yesterday's price reductions, Intel is now releasing a new 45nm dual-core processor. Previously reported in some Intel price charts, Intel Core 2 Duo E8300 is a 1333MHz FSB CPU with 6MB of L2 cache, a 65W TDP and stock frequency of 2.83GHz. The CPU has already made its way into stores with a price of $163 in 1000-unit tray quantities.

The E8500 competes quite well against the Q6600, and even stacks up against the QX9550, so unless you're doing something involving heavy multithreading, I'd go for Dual Core.
Why does Intel occasionally skip a number? Like, the E6400 and E6600, with no E6500. Only later did the E6550 come out, with a higher FSB.

Intel skips the models to prevent there being too many choices. A dell user who has 20 options will just give up and go for the cheapest one, whereas 2-3 choices works best. Later on as AMD catches up with a competitive model, or the sales drop down they will release a new fill in model to entice new users.

think like this:

If they have model A B and C (2Ghz 2.4GHz and 3GHz) the high end users will get C, the low end users get A, and those undecided go for B. If they then release a2 b2 and c2 (2.2, 2.6 and 3.2) then many people (to outdo friends, to get 'faster', etc) will go grab those instead - regardless of any real performance increase, they just want something that looks better. If intel only released C2, then people tend to get bored with the same product being out for ages and try something else.

moral of the story: incremental upgrades mean people essentially buy the same product over and over again - more profit.